[Jetcaster]: Change alpha of prev/next buttons. (#1393)

Change alpha of previous and next buttons in the player to help indicate
if those actions are possible based on the state of the player (i.e.
previous only possible while playing and next only possible when queue
isn't empty).

Disabled:
<img
src="https://github.com/android/compose-samples/assets/463186/81bf2fcb-c8af-4db3-bf79-421c79b2393f"
width=300 />


Enabled:
<img
src="https://github.com/android/compose-samples/assets/463186/f6cfebbf-e54e-41bf-9813-302536a1cb5e"
width=300 />

Jetcaster/mobile/src/main/java/com/example/jetcaster/ui/player/PlayerScreen.kt

@@ -74,6 +74,7 @@ import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.alpha
 import androidx.compose.ui.draw.clip
 import androidx.compose.ui.graphics.ColorFilter
 import androidx.compose.ui.layout.ContentScale
@@ -754,6 +755,7 @@ private fun PlayerButtons(
             colorFilter = ColorFilter.tint(MaterialTheme.colorScheme.onSurfaceVariant),
             modifier = sideButtonsModifier
                 .clickable(enabled = isPlaying, onClick = onPrevious)
+                .alpha(if (isPlaying) 1f else 0.25f)
         )
         Image(
             imageVector = Icons.Filled.Replay10,
@@ -807,6 +809,7 @@ private fun PlayerButtons(
             colorFilter = ColorFilter.tint(MaterialTheme.colorScheme.onSurfaceVariant),
             modifier = sideButtonsModifier
                 .clickable(enabled = hasNext, onClick = onNext)
+                .alpha(if (hasNext) 1f else 0.25f)
         )
     }
 }